National Instruments AT-MIO-64F-5, User Manual

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Chapter 6 Calibration Procedures
© National Instruments Corporation 6-7 AT-MIO-64F-5 User Manual
All these error sources may be calibrated without making any connections to the AT-MIO-64F-5.
A properly calibrated board will be accurate in both bipolar and unipolar modes without
adjustment.
Pregain offset contributes gain-dependent error to the analog input system. This offset is
multiplied by the gain of the PGIA. To calibrate this offset, the routine should ground the inputs
of the PGIA, measure the input at two different gains in bipolar mode, and adjust CALDAC0
until the measured offset in LSBs is independent of the gain setting.
Postgain offset is the total of the voltage offsets contributed by the circuitry from the output of
the PGIA to the ADC input (including the ADC's own offsets). To calibrate this offset, the
routine should ground the inputs of the PGIA, measure the input at two different gains in bipolar
mode, and adjust CALDAC1 until the measured offset is proportional to gain setting.
Unipolar offset is additional postgain offset that is present only in unipolar mode. It is due to
inaccuracy in the circuitry that switches between bipolar and unipolar modes. To calibrate this
offset, the routine should ground the inputs of the PGIA in bipolar mode and adjust CALDAC1
to yield a small positive measured offset (typically, two or three LSBs). Then it should switch
the board to unipolar mode and adjust CALDAC2 to yield the same offset in LSBs as that
measured in bipolar mode. Finally, CALDAC1 should be restored to its previous value.
If the three offset DACs are adjusted in this way, there is no significant residual offset error, and
reading a grounded channel returns (on average) less than ±0.5 LSB, regardless of gain setting.
All the stages up to and including the input of the ADC contribute to the gain error of the analog
input circuitry. With the PGIA set to a gain of 1, the gain of the analog input circuitry is
ideally 1. The gain error is the deviation of the gain from 1 and appears as a multiplication of the
input voltage being measured. To eliminate this error source, the routine should measure the
input first with the inputs grounded and then with the inputs connected to the internal voltage
reference. It should then adjust CALDAC3 until the difference between the measured voltages is
equal to the value of the reference as stored in the onboard EEPROM. Once the board is
calibrated at a gain of 1, there is only a small residual gain error (±0.02% maximum) at the other
gains. The gain adjustment may have a small effect on postgain offset and unipolar offset, so for
best results gain should be calibrated before postgain offset and unipolar offset.
Analog Output Calibration
To null out error sources that affect the accuracy of the output voltages generated, the output
calibration routine should calibrate the analog output circuitry by adjusting the following
potential sources of error:
¥ Analog output offset error
¥ Analog output gain error
Both of these error sources may be calibrated without making any connections to the
AT-MIO-64F-5. However, the errors will differ between unipolar and bipolar modes, so
separate calibrations will be necessary for each mode.